The latter is derived for modeling propagation of nonlinear pulses in monomode optical fibers
where \(u\)  is assumed to describe the slowly varying envelope of the pulse [1]}.
It is interesting that Eq.(REF ) is a reduced potential form of the first negative member in the KN hierarchy,
\(& u_{xt}+u-2iuvu_{x}=0,\\& v_{xt}+v+2iuvv_{x}=0,\) 
